The present invention relates to atrium-controlled heart pacemakers. Such pacemakers generally include a pulse generator operating synchronously with the heart P-waves. The pulse generator operates continuously between a programmed base frequency and a maximum frequency. In pacemakers of this type, a closed loop is sometimes created by the atrial lead and detector, the ventricular output stage and a lead retrograde conduction path in the heart tissue from the ventricle to the atrium. Such a closed loop presents conditions under which pacemaker-induced tachycardia may occur.
It is an object of the present invention to provide an atrium-controlled heart pacemaker wherein pacemaker-induced tachycardia may be automatically terminated.
The above object is achieved in accordance with the principles of the present invention by an atrium-controlled heart pacemaker which includes a monitoring circuit for the stimulation circuit which interrupts forwarding of the P-wave to the stimulation circuit when the frequency of P-waves exceeds a predetermined value. The monitoring circuit includes a window generator which sets a window within which a predetermined maximum number n of P-waves are allowed to occur. When the n.sup.th P-wave occurs within the window, this is an indication that tachycardia induced by the heart pacemaker is present. Under such circumstances, the generation of a stimulation pulse on the basis of the next P-wave acquired by the atrium electrode is suppressed, and thus the closed circuit is interrupted and the tachycardia may be terminated.